450 THE PRINCIPLES OF SCIENCE. 



same laws as regards temperature, pressure, and volume. 

 But in reality these laws are only approximately true, 

 and the divergences have afforded a wide and yet quite 

 unexhausted field for further generalization. The more 

 recent discoveries of Cagniard de la Tour and Professor 

 Andrews might seem to have summed up many of these 

 exceptional facts under a wider generalization, but in 

 reality they have opened to us vast new regions of in- 

 teresting inquiry, and they leave wholly untouched the 

 question why one gas or one substance behaves differently 

 from another. 



The science of 'Crystallography ^s that perhaps in which 

 the most precise and general laws have been detected, but 

 it would be utterly untrue to assert that it has lessened 

 the area of future discovery. We can show that each one 

 of the seven or eight hundred forms of calcite is derivable 

 by plain geometrical modifications from an hexagonal 

 prism, but who has attempted to explain the molecular 

 forces producing these modifications, or the chemical con- 

 ditions in which they arise^ The law of isomorphism is 

 an important generalization, for it establishes a general 

 resemblance between the forms of crystallization of natural 

 classes of elements. But if we examine a little more 

 closely we find that these forms are only approximately 

 alike, and the divergence peculiar to each substance is an 

 unexplained exception. 



By many similar illustrations it might be readily shown 

 that in whatever direction we extend our investigations 

 and successfully harmonize a few facts, the result is only 

 to raise up a host of other unexplained facts. Can any 

 scientific man venture to state that there is less opening 

 now for new discoveries than there was three centuries 

 ago ? Is it not rather true that we have but to open a 

 scientific book and read a page or two, and we shall in all 

 probability come to some recorded phenomenon of which 



